1. Field of the Invention
The present invention generally relates to silicon-controlled rectifiers (SCRs), and more particularly to SCRs with increased holding voltage and to polysilicon resistance triggered stack SCRs.
2. Description of Related Art
A silicon-controlled rectifier (SCR) is a solid state current controlling device and a bidirectional device adopted as an electrostatic discharge (ESD) device. FIG. 1 shows a current-voltage curve of a typical SCR concerning ESD in an integrated circuit (IC). In general, a trigger voltage should be less than a safe voltage Vsafe, which assures no breakdown to an oxide layer in electrostatic discharge. On the other hand, a holding voltage should be higher than maximum rated device voltage Vddmax, which assures no latch-up to a normally operated device.
Some schemes have been proposed to increase the holding voltage in order to prevent latch-up. However, those schemes may incur current crowding and decrease the current value at the second breakdown, thereby sacrificing ESD capability. Moreover, those schemes may disadvantageously increase the trigger voltage.
When the conventional SCRs are connected in series, the holding voltage and the trigger voltage become proportional to the amount of the SCRs. A scheme has been proposed by using a guard ring. However, the trigger voltage may increase to some extent when the amount of the connected SCRs increases.
A need has thus arisen to propose a novel scheme to overcome the disadvantages of the conventional SCRs.